Glass having an expansion lower than that of silica



Ag 3, 1943' M. E. NORDBERG 2,326,059

GLASS HAVING AN EXPANSION LOWER THAN THAT OF SILICA Filed April 22, 1939 'ri""arlllrif'p'l'11-111111' Pressure n mm.

0 90 a" cx, N01; 72, c/4 rf a4 EWE/( Ar RM1-3?.

Patenfed Aug. 3, 1943 i Y 2,326,059

- UNITED STATES PATENT OFFICE GLASS IAVING AN EXPANSION THAN THAT 0F SILICA Martin E. Nordbergf,y Corning, N. Y., assignor to Corning Glass Works, Corning, N. Y., a corporation of New York Application April 22, 1939, Serial No. 269,497 s claims. (C1. 10s-52) This invention relates to glasses having low pared together with their expansion coeiiicients expansion coeflicients and high silica contents as measured will illustrate my invention: and has for its primary object to produce a glass f having a coeicient of expansion less than that 1 n m 1v v of pure fused silica. 5

Another object is to produce a glass having an sm, 94.7 93.8 91 e 90.3 89.6 expansion coemcient which is substantially zero l2 M 9-.7 -4

in the temperature range irom 0 C. to 300 C., that is, a. glass which will not expand when heated in this range. A

A further object is to produce an opal glass having the above `described desirable properties.

With the above and other objects in view, the invention consists in the glass and the method of making it, to be hereinafter more fully del5 ein einer; 1n above of silica and over 5% but not over 11% of titanium Smau amounts of mama' o? suma' muy be replaced oxide hereinafter called ,mama by aluminum oxide or zirconium oxide in an The invention further comprises the method 2o amount not to exceed about 5% Without causing which includes vaporizing the tetrachlorides of substantial change in the valuable properties oi silicon and titanium into the gas stream of an the'glasses' The glasses of my invention are very refractory ox as burner, de ositin the resultant mixture of lgxgdes to make prefcm Vitrifying the pre and reqmre suchhigh temperatures for melting form at about l500 C. to make an opal glass 25 them that the usual glass melting methods do thereof and ring the opal preform at a higher p got prudce Satisfacrhresulod hey. y temperature to cause it to become transparent. dowey .epreparet y .ehnl emma er i have discovered that under proper conditions, esem m comme on WH e accompanying to be hereinafter more fully set forth and dedrawing in which:

Fig. 1 is a side view partly in section of an scribed, titanio. and silica in Aa certain range of 3o proportions can be combined to formI a series apparatus for formmg and depositmg a' mixture of glasses which are transparent or opal as de of oxides suitable for making a glass in accorda 'th my invention; sired, and which have very low expansion coeine? W1 cients. With titana contents between about 5% Flg' 2 1s a Side View partly m section of u modi' It will be noted that the expansion coeiclents of the above glasses are below that of fused silica, which is generally accepted to be about 5.5 X 10". Glasses 1H and IV have an expansion coecient which is substantially zero.

and 11% the expansion coemcients of the result- 35 ed form of apparatus for generating mixed Y ing glasses are substantially less than that of vapors oi silicon and titanium tetrachlorides;

h lo es ex ansion c0 Fig. 3 is a side view partly in section of another gnhh sestne gertofre knowno alternative form of generator for the vapors of 'When the titania content is increased to about Silicon and titanium tetrachlorides;

9%, the expansion coemcient becomes substan- 4o Fig' 4 is a' side View partly in section of un tially zero insofar as can be determined by the apparatus for generating the vapor of aluminum usual methods, that is, a glass of this composition chloride' or zirconium chloride;

does not appreciably expand nor contract when Fg- 5 is a perspective sectional View of a' fur' heated or cooled in the temperature range 0 C. Pace Showing a preformed mixture of Oxides be' to 300 o. when the titel-11a. content is increased 45 me ,Vitriied and beyond about 9%, the expansion coeiiicient in- F1g 6 1S a graph showing the relationship becreases rapidly and with a titania content greater tween Calculated Vapor pressure and molecular than about 11% the expansion coemcient is too percentage 0f liquid mixtures 0f Silicon tetralarge'for my purpose and the glass is otherwise chloride and titanium tetrachloride at 20 C. and unsatisfactory. 5o the vapors in equilibrium therewith.

The following compositions which I have pre In Fig. 1 a vapor generator generally designated l comprises a ask ii containing a liquid mixture oi silicon tetrachloride and titanium tetrachloride which is 'supported in a constant temperature bath `I2 and is provided with ai tube I3 for introducing a combustible gas and a delivery tube I4. A by-pass valve l5 connects the tubes i3 and I4. The tube I4 is further connected through a reducing valve I3 with an oxygen tank 'l1 and with a burner tip I3. In/front of the burnerv tip I3 a tapered mandrel i3 composed preferably of nickel or other non-scaling refractory metal is held by a rotatable chuck 20 which is actuated by an ordinary drill press (not shown) so that the mandrel I3, while rotating, may be reciprocated through the flame in a direction parallel with the axis of the mandrel.

In Fig. 2 a ilask 2| supported in a heating bath 22 and a flask 23 .supported in a heating bath 24`are joined in series with -a gas line 25 and a delivery tube 26. A by-pass valve 21 connects the line 25 and the tube 26.

In Fig. 3 a ask 28 supported in a heating bath 23 and a ask 30 supported in a heating bath 3l are joined inparallel relation with a gas line 32 and a delivery tube 33 and are provided with valves 34 and 35. A by-pass valve 36 connects the 'line 32 and the tube 33.

In Fig. 4 a vapor generator generally designated 31 comprises a cylindrical metal barrel 33, within which are disposed a refractory boat 33 and bailles 40. lThe barrel 38 is provided at one end with a burner tip 4| and at the other end with a removable cover plate 42 and is surrounded by a heating element 43 and refractory insulation 44. 'I'hrough the plate 42 a metal tube containing a thermometer 46 projects into the barrel 38 and is closed at its inwardly projecting end. A tube 41 for introducing combustible gas and vapors extends through the plate 42 and communicates with the interior oi the barrel 38.

In Fig. 5 a preform 48'is suspended from a platinum wire 49 within an electric furnace generally designated 50 and comprising a tubular core 5i provided with a platinum heating element 52 and thermal insulation 53.

For the preparation of my glasses by means of the above described apparatus a combustible gas, such as natural gas, is introduced through the tube I3 and ignited at the burner tip I8. By means of the reducing valve i6 a controlled amount of oxygen is admitted to the burner from the tank l1 to produce a high temperature flame. A mixture of the vapors of silicon and titanium tetrachlorides is then introduced into the gas stream by closing the by-pass valve l5 and on passing through the llame the vapors are decomposed by the heat and the combustion products to form an anhydrous mixture of the oxides of silicon and titanium. The oxide mixture carried by the flame is deposited on the rotating mandrel I9 whereit builds up a tubular, coherent,-

amorphous deposit or preform. After the preform has attained a suitable thickness, say 2 mm. or more, it is allowed to cool and is removed from the mandrel after which the preform s vitried by heating it to l500 C. or higher, as will later be described.

The composition of the preform maybe predetermined and is controlled preferably by using in the generator I0 a liquid mixture of silicon tetrachloride and titanium tetrachloride in the proportions rwhich will evolve at a given temperature a mixture of the vapors of the salts which is equivalent to the oxide composition of the desired glass. The proper proportions oi the liquid mixture may be determined most conveniently by means of curves or graphs representing the q relationship between vapor pressure and composition at the chosen temperature. Using the known vapor pressures of pure silicon and titanium tetrachlorides .and assuming that the mixed liquids and their vapor pressures conform to the laws for ideal solutions and gases, the curves in Fig. 6 were constructed as an Yexample and can be used to show such a relationship for these substances at the arbitrarily selected temperature of C. Curve A shows how the vapor pressure of a'liquid mixture of these substances at 20 C. varies with the molecular composition of the mixture and curve B shows the molecular compositions of the various vapor mixtures which are in equilibrium with these liquid mixtures.

In determining the proportions of the materialsy to be used, the percentage of silica and titania in the desired glass are converted by calculation to the equivalent molecular percentages of silicon tetrachloride and titanium tetrachloride. This represents the composition of the vapor mixture which must be employed. From curve B the vapor pressure of the corresponding liquid mixture 1s ascertained and from curve A the molecular proportions of this liquid mixture ls found. By conversion of the latter figures to the weight percentage basis the composition of the required liquid mixture in percent by weight is determined. t

For example, in case it is desired to produce a glass having the percentage composition SiOa and. 10% TiOr, the equivalent molecular percentages of the chlorides corresponding thereto is 92.2 mol SiCl4 and 7.8 mol TCli. From curve B it is found that the vapor pressure at 20 C. of a liquid mixture the vapor of which has this composition, is about 90 mm. and from curve A it is seen that the composition of this liquid mixture is 62 mol TiCl4 and 38 mol SiCli which is equivalent to 64.5% TiCli and 35.5% SiCl4 by weight. In a similar manner the compositions of all liquid mixtures of silicon chloride and titanium chloride which can be used for the production of my glasses may be determined.`

Since the composition of the vapor removed will be different than the composition of the liquid, the composition of the latter will gradually change as the vapor is used. If the initial volume of the liquid mixture is sufliciently large, this change will be inappreciable for a substantial length of time, but if the initial volume is small the change will be relatively rapid. In order to maintain the proper liquid composition, it is necessary to replenish the liquid mixture by the constant addition of fresh liquid having the composition of the vapor mixture which is being withdrawn and equivalent thereto in amount. Means for making such addition canreadily be devised by one skilled in the art.

If desired, the liquid silicon tetrachloride and titanium tetrachloride may be vaporized from separate containers and the vapors .thereafter mixed by introduction into the gas stream. For example, in lieu of the generator I0 there may be substituted the series generator shown in Fig. 2 or the parallel generator shown in Fig. 3.

In using the series generator, the silicon tetrachloride is placed in the flask 2l and the titanium tetrachloride is placed in the flask 23. The amount of vapor introduced into the gas stream 1s governed both by the amount of combustible gas passed through the liquids, which in turn is controlled by the by-pass valve 21, and by the temperatures of the liquids. The proportions of the two vapors is controlled by the relative temperatures of the two liquids as maintained by the heating baths 22 and 2t, the temperature of the latter being always above'the boiling point of silicon tetrachloride.

In using the parallel generator of Fig. 3 the silicon tetrachloride and titanium tetrachloride 'are placed individually in the flasks 28 and 3i) and the amount of combustible gas passed through each ask is controlled by the valves it and 35 respectively. 1n this case the relative proportions of the two vapors is controlled not only by the temperatures at which the two liquids are maintained by the heating baths 2Q and 3l,

but also by the relative amounts of gas passed through each nask. The same 'result may be obtained by using high pressure containers such as steel bottles in lieu-oi the two asks and maintaining each at a sufficiently high temperature to generate therein a high vapor pressure, the amount of vapor introduced into the gas stream from each container being controlled and measured by means of valves and owmeters.

It, as hereinbefore mentioned, it is desired to introduce small amounts of other oxides, such as aluminum oxide or zirconium oxide, into the silica-titania composition of the preform, this y may readily be accomplished by additionally introducing into the gas stream the vapors of their chlorides. For this purpose the vapor generator ill, shown in Fig. 4, is substituted in lieu of the burner tip it in Fig. 1. Aluminum chloride or zirconium chloride is placed in the refractory boat 39 and heated sufficiently by means of the heating element 43 to cause vaporization of the salt at a desired rate. The rateoi vaporization is controlled by controlling. the temperature of the barrel l i through the power input of the electrical heating element 43.

The method of making glass by decomposing mixtures of gaseous glass-forming compounds in a high temperature flame and vitrifying the resultant oxide mixtures is broadly claimed in another application Serial Number 269,496 filed jointly by me and Robert H. Dalton of even date herewith.

My glasses containing silica and titania can be made either clear and transparent or opaque,

that is, opal, depending upon the manner in which the preform is vitrined. if vitrication is accomplished at about 1500 C., the resulting glass is a dense opal but, ii.' the opal glass is subsequently heated at about 1600 C. or above, the opacity disappears and the glass becomes clear and transparent. Likewise, if the amorphous preform is heated directly at a temperature of about 1600'C. or above, the resulting glass' is clear and transparent. My glasses, when once they have been made clear, either by a preliminary opalization at about 1500 C. followed by a subsequent heating at higher temperature or by a direct heating at the higher temperature, cannot again be converted to a good opal glass by reheating at 1500 C. This phenomenon is characteristic of .my new compositions and is not exhibited by any prior glasses insofar as I am aware.

On account of an appreciable shrinkage of the preform on firing with a consequent possibility of cracking if heated too rapidly and since a reducing atmosphere is objectionable in firing, vitriiication is best accomplished by introducing the preform slowly into an electric furnace or gas iired muiile with an oxidizing atmosphere. For example, as shown in Fig. 5, the preform 48 is suspended by the platinum wire d@ in such. a manner that it can be lowered gradually into the electric furnace 5@ which in its hottest region is maintained at a temperature of about 1500 C. The rate at which the preform is lowered should be slow enough to allow the air in the pores thereof to be expelled as the oxides fusetogether. About fteen minutes should suifice to pass a preform six inches long through the furnace. Under these conditions the resulting glass will be a dense opal. On account of` its extremely low expansion the preform when once it has been vitrified can be heated or cooled as rapidly as desired. By a subsequent heating in a high temperature flame or inthe electric furnace maintained at a temperature of 1600 C. or above, the opacity of the glass disappears and it becomes very clear and transparent and possesses a high lustre. If desired, the glass can be' made clear on the first heating by using a. temperature of 1600 C.or above.

'I'he glass thus made can be reworked in a high temperature ame and molded and formed into desired articles in the manner known to those who are skilled in such manipulations. Articles which are to be opal must be preformed as above described because they cannot be subsequently heated to softness without losing their opacity.

I claim:

1. A glass which consists of 89%-95% of silica and over 5% but not over 11% of titanium oxide and which has a. coemcient, of expansion less than that of silica.

2. A glass which consists of silica and titanium oxide, the titania being over 5% but not over 11% the glass being substantially transparent.

3. A glass which consists of silica and titanium oxide, thetitania being over 5% but not over 11%, the glass being opal.

MAR'I'IN E. NORDBERG. 

